Implement type inference for try/catch blocks.

pkg/front_end/testcases/inference/try_catch.dart

+// Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+/*@testedFeatures=inference*/
+library test;
+
+class C {}
+
+class D {}
+
+class E {}
+
+void test(void f()) {
+  try {
+    var /*@type=int*/ x = 0;
+    f();
+  } on C {
+    var /*@type=int*/ x = 0;
+  } on D catch (x) {
+    var /*@type=D*/ x2 = x;
+  } on E catch (x, y) {
+    var /*@type=E*/ x2 = x;
+    var /*@type=StackTrace*/ y2 = y;
+  } catch (x, y) {
+    var /*@type=dynamic*/ x2 = x;
+    var /*@type=StackTrace*/ y2 = y;
+  }
+}
+
+main() {}